Environmental concerns have recently stimulated the coating industry to find ways to reduce pollution by volatile organic compounds used as solvents in painting and finishing operations. A great deal of emphasis has been placed on the development of new coating technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged to meet most but not all of the performance and application requirements, and at the same time meet emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids compositions. Each technology has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that was hoped for initially.
Powder application, for example, while providing ultra-low emission of organic vapors, is oftentimes characterized by poor gloss or good gloss with heavy orange peel, poor definition of image gloss, and poor film uniformity. Pigmentation incorporation is often a problem that at times requires milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coatings line often requires a complete cleaning because of dust contamination of the application equipment and finishing area.
Water-borne coatings cannot be applied under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic co-solvents of the coalescing aid. As might be expected, in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation resulting in poor gloss, poor uniformity and pin holes. Additionally, water-borne coatings are not as resistant to corrosive environments as are the more conventional solvent-borne coatings.
Coatings applied from organic solvents at high solids avoid many of the pitfalls of powder and water-borne coatings. In these solvent systems, the molecular weight of the polymer is often low and reactive functionality is necessary to crosslink the coating after it has been applied. In reality, there is a limit in the ability of this technology to meet the performance requirements of a commercial coating operation. Present high solids systems are difficult to apply to vertical surfaces without running and sagging of the coating. Often they are prone to cratering and pin-holing. If they have good reactivity they have poor shelf and pot-life; if they have shelf stability then often they cure and/or crosslink slowly or require high temperature and energy to cure.
The use of supercritical fluid as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. The applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describe forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing fine films and powders. The films are used as surface coatings.
U.S. patent application Ser. No. 133,068, now abandoned and U.S. Pat. No. 4,923,720, to Lee et al., disclose a process and apparatus for the liquid spray application of coatings to a substrate while minimizing the use of the environmentally undesirable organic diluents. The process of the application involves forming a liquid mixture comprising a coating polymer and a supercritical fluid sufficient to render the viscosity of said mixture suitable for spray application and then spraying the liquid mixture onto a substrate to form a liquid coating. The application and patent describe the addition of at least one organic solvent to the liquid mixture prior to spraying. The preferred supercritical fluid is supercritical carbon dioxide. The process employs an apparatus in which the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus contains multiple means for supplying at least one polymeric coating compound, for supplying at least one organic solvent and for supplying supercritical carbon dioxide fluid and means for forming a liquid mixture of these components. These means are combined with means for spraying said liquid mixture onto a substrate. The apparatus may contain means for heating any of said components and/or said liquid mixture of components. U.S. patent application Ser. No. 133,068, now abandoned and the patent demonstrate the use of supercritical fluid, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous nonaqueous dispersions coatings compositions to dilute the compositions to application viscosity required for liquid spray techniques. They further demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
European Patent Application 89 112823.3, published Jan. 17, 1990, is directed to a liquid coatings application process and apparatus in which supercritical fluid, such as supercritical carbon dioxide fluid, are used to reduce to application consistency viscous coatings compositions to allow for their application as liquid sprays. The coatings compositions are sprayed by passing the composition under pressure through an orifice into the environment of the substrate.
European Patent Application 89 112856.3, published Jan. 17, 1990, is directed to a process and apparatus for coating substrates by a liquid spray in which 1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations, 2) the mixture of supercritical fluid and coating formulation is passed under pressure through an orifice into the environment of the substrate to form the liquid spray, and 3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
The above technologies amply demonstrate the applicability of supercritical fluids as carriers and viscosity reducers for transporting a variety of coating materials and effectively spraying them onto a coatable surface while reducing the amount of volatile organic compounds (VOCs) that are required for the application.
European Patent Application 90 105347.0, published Sep. 26, 1990, also relates to the use of supercritical fluids to replace VOCs is spray coating compositions. It is there indicated that low molecular weight polymer component makes it easier to atomize in a spray gun. To avoid the presence of elastic component of viscosity, the EP application recommends the use of polymers having a M.sub.w /M.sub.n less than about 4, preferably less than about 3, and most preferably less than about 2. Table 2 of the EP publication describes the use of acrylic thermosetting oligomers having a M.sub.n of 1,000-2,000, a M.sub.w of 1,200-2,200, and a M.sub.w /M.sub.n of 1.1-1.5.
The molecular weight of a polymer controls many of its physical properties, such as modulus, yield strength, fracture toughness, viscosity and solubility. As the molecular weight of a polymer is increased, most of these properties increase to a maximum and then plateau, whereas the solubility of a polymer decreases as its molecular weight is increased. Since many coatings are sprayed from solutions, their molecular weights must be chosen to balance solvent compatibility, solution viscosity and the required mechanical properties.
It is recognized that short chain acrylic oligomers are deficient in properties required of a useful commercial coating unless the acrylic oligomer is thermosetting by being capable of being crosslinked. Oligomers of this type, containing up to about 15 monomeric units.sup.1 posses properties distinctive from a corresponding higher molecular weight polymer. It is through a crosslinking reaction that effective coating properties are achieved using these oligomers. Polymers having more monomer units than an oligomer pass through a structural phase that introduces significant physical and chemical properties than would not be present in the corresponding oligomer. FNT 1. The definition of oligomer is not a fixed one. According to the Encyclopedia of Polymer Science and Technology, vol. 9, page 485: FNT "Oligomers are the low-M.sub.w, or low-DP, members of a homologous series, the high-M.sub.w, of high-DP, members of which are polymers.-- . . . oligomers are molecularly homogeneous polymer homologs of low molecular weight which show differences in their physical properties sufficiently clear to distinguish among them. Oligomers can in addition be isolated from their mixtures as chemical entities using classical fractionating methods." FNT The Polymer Handbook, 2nd Edition, page VI-2, states- FNT "Oligomers are defined as the low members of the polymeric-homologous series, with molecular weights up to about 1000-2000." FNT Vilenchik et al., Acta Polymerica, 36 (1985) Nr. 3, state: "On the natural physical boundary between oligomers and proper polymers in the hydrodynamic behavior of macromolecules" indicate that the number of repeating units of an oligomer is in the range of 10-15 units.
The molecular weight distribution of a polymer also influences many of its physical properties and is often described in terms of polydispersity (M.sub.w /M.sub.n). In a typical free radical polymerization reaction, polymers with a heterogeneous molecular weight (polymer chains of different lengths) are produced. These systems are termed polydisperse, where M.sub.w /M.sub.n typically ranges from about 2 to about 4. Anionic polymerization reactions (generally conducted at temperatures below O.sup.0 C) yield polymer chains of essentially the same length. These systems are termed monodisperse, where M.sub.w /M.sub.n typically ranges from slightly above 1.0 to about 1.5. Such control of M.sub.w /M.sub.n has been known to permit useful variation in polymer physical properties, such as glass transition temperature, hardness, heat distortion temperature, and melt viscosity.
Anionic polymerization is recognized to allow synthesis of polymers having a predictable molecular weight average and a very narrow molecular weight distributions, approaching the Poisson distribution. Morton, Anionic Polymerization: Principles and Practice, Academic Press, New York, 1983, page 9. Morton, at pages 169-174, discusses monodisperse polymers, and at pages 174-176, list a substantial number of polymers formed by anionic polymerization that possess narrow molecular weight distributions.
Group transfer polymerization (GTP) is an anionic polymerization technique for preparing monodisperse polymers at more moderate temperatures, e.g., room temperature. .sup.2 According to Webster, .sup.3 GTP is useful for producing "polymers [of]. . . low molecular weight and having a . . . narrow molecular weight distribution (M.sub.w /M.sub.n). At a given temperature, M.sub.w /M.sub.n is primarily a function of the relative rates of initiation and polymerization. Rate of initiation, r.sub.i, depends on initiator and co-catalyst type and relative concentrations. Polymerization rate, r.sub.p, is a function of monomer reactivity and co-catalyst type and concentration. For monodispersity, r.sub.i /r.sub.p is equal to or greater than 1, that is, the initiation rate is at least as fast as the polymerization rate and all chains grow simultaneously.".sup.4 FNT 2. See: Webster, O. W., "Living Polymers And Process For Their Preparation", U.S. Pat. No. 4,417,034, Nov. 22, 1983. FNT Farnham, W. B and Sogah, D. Y., "Process For Preparing Living Polymers", U.S. Pat. No. 4,414,372, Nov. 8, 1983. FNT 3. See footnote 2. FNT 4. Webster, supra, indicates that GTP has "the ability to copolymerize methacrylate and acrylate monomers, or combinations of acrylate monomers, for example, ethyl and sorbyl acrylates, to relatively monodisperse copolymers."
Narrow molecular weight polymers have significant advantages for use in solvent based coatings. The absence of low molecular weight polymer fractions gives cured coatings with better physical properties. In addition, the absence of high molecular weight fractions, as pointed out herein, allows for the preparation of high solids solutions from these polymers which have lower solution viscosities than do solutions containing polymers of the same average molecular weight prepared by free radical polymerization.